The camera in a mobile phone or tablet platform is typically referred to as a camera module. Traditionally, a camera module consists of a single sensor and a single lens producing a single image of the scene with each exposure. FIG. 1 illustrates a camera module that consists of an image sensor (typically CMOS) mounted to a substrate, a lens and a lens holder. The CMOS sensor can be in a pre-packaged form or a bare-die which is then bonded to the substrate in a highly clean environment. Once the sensor is attached to the substrate, the lens holder is precisely placed such that the optical axis of the lens matches with center of active sensor array. Once the lens holder is in place and glued, the lens is attached to the lens holder and threaded in. The focal position of the lens is adjusted in order to maximize the module modulation transfer function (MTF) across various field positions and a couple of object positions. The factories manufacturing the cameras module have finely tuned this process and are able to consistently produce high quality modules with higher than 90% yield and excellent throughput. The net result is that the camera module has high mechanical tolerances and consistent optical performance.
The camera-array or multi-camera system is a new area of development in imaging. Such systems can simultaneously capture multiple views of the scene using multiple lenses. Having multiple views of the same scene enables the system to extract depth of the objects in the scene using the parallax between the various views. In the past, a camera-array module was built using a single custom-designed CMOS sensor on a monolithic piece of silicon. These are custom-designed sensors include sub-arrays, with each sub-array having their own control and read-out circuits. FIG. 2 shows a multi-camera array using a monolithic sensor and a single lens with lens-lets to match each sub-array unit in the sensor.
There are a number of problems associated with manufacturing an array camera. First, in order to build a camera-array module, the array sensor is assembled with a lens-array which is a single unit with multiple lenses. Each sub-array has its own lens-let such that the optical center of the sub-array and lens-let are matched. In order for the all the sub-modules to have consistently good focus, the factories have to develop new ways to focus the lens and test the modulation transfer function (MTF) of each sub-camera module. One of the methods for this purpose used is referred to as “active alignment,” in which the lens position is actively controlled in the x, y and z directions and when the correct position is found the lens is glued in-situ and snap-cured. This process requires high precision stations and the takes significantly more time compared to a traditional module assembly step, thus reducing overall assembly line through-put. The array module yield is also low since the sub-par performance of a sub-array in the entire array will force the entire module to be rejected. There is limited scope to re-work a selected sub-array given how the module is put together. As a result, the array module assembly requires new capital equipment and changes to established module assemblies work flow, which adds to the module cost.